The need is felt to provide surveillance of crowded areas (for example, airport or sea-port terminals, railway stations, large department stores, shopping centres, industrial plants, etc.) within which people move in order to detect in a timely way the presence of toxic agents in the air present in said area. In these cases, a first-alarm sensor is required that will have the highest likelihood of detection of the substance.
Said toxic agents can be released in the air following upon failures (for example, failures in an industrial plant) or else willful damage such as acts of terrorism.
To meet the need for surveillance referred to above currently used are networks of point sensors (for example, chemical sensors or optical sensors) designed to detect the presence of toxic agents in circumscribed points of the area under surveillance.
Generally these solutions present some limits:                the use of point sensors entails complexity and high costs since it is necessary to use a large number of interconnected parts;        the majority of point sensors used does not guarantee a continuous monitoring (for example, they interact with the environment, and require passage from a sampling mode to a cleaning mode);        point sensors of an optical type can guarantee continuous operativeness, sensitivity and selectivity, but in order to enable all these features to be provided simultaneously they prove too costly to enable their use in a large number.        